This invention relates to input/output (I/O) controllers and microcontrollers for transferring data between a host processor and one or more peripheral input/output (I/O) devices in a digital data processing system.
It has been heretofore proposed to employ microprocessors as I/O controllers for transferring data between host processors and I/O devices. In such cases, the data is transferred from the host processor into the microprocessor and then from the microprocessor to the I/O device for an I/O device "write" operation and, conversely, from the I/O device into the microprocessor and then from the microprocessor to the host processor for an I/O device "read" operation. This is somewhat time consuming in that numerous instructions must be executed inside the microprocessor for each byte or word of data transferred. It is a purpose of the present invention, therefore, to provide a new improved automatic high-speed bypass mechanism whereby, when operating in the automatic mode, data may be transferred directly from the host processor to the I/O device, or vice versa, without having to pass through the microprocessor and without requiring any intervention on the part of the microprocessor in the absence of errors or malfunctions.
It is another purpose of the present invention to provide a new and improved I/O controller for enabling a host processor to communicate with a plurality of I/O devices in a more rapid and efficient manner. In particular, in the embodiment described herein, the controller includes mechanisms for enabling servicing of an I/O command from the host processor for one I/O device while high speed data transfers in a cycle steal mode are taking place for a second I/O device and while the microprocessor is initiating an interrupt sequence to the host processor for a third I/O device.
For a better understanding of the present invention, together with other and further advantages and features thereof, reference is made to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.